Is expansion an emergent property?

[Genady]

Would it be correct to say that cosmological expansion is an emergent property? I think so because it appears only on scales of hundreds of megaparsecs where the space becomes homogeneous and isotropic on average.

[TheVat]

Good question.  How would a vacuum energy model of expansion work with emergentism?  Though near objects are gravitationally bound, a vacuum energy still weakly repels and so all objects are slightly larger than they would be.  Then expansion would be intrinsic?  So not emergent?  

[Genady]

9 minutes ago, TheVat said:

Good question.  How would a vacuum energy model of expansion work with emergentism?  Though near objects are gravitationally bound, a vacuum energy still weakly repels and so all objects are slightly larger than they would be.  Then expansion would be intrinsic?  So not emergent?  

These need to be considered separately. Vacuum energy is not a model of expansion. GR alone models expansion. Vacuum energy models acceleration of expansion.

And in fact, effect of vacuum energy on the cosmological scales would be acceleration of expansion while on lab scales it is something else, an extra repulsion unrelated to any expansion.

[studiot]

4 hours ago, Genady said:

Would it be correct to say that cosmological expansion is an emergent property? I think so because it appears only on scales of hundreds of megaparsecs where the space becomes homogeneous and isotropic on average.

emergent from what ?

As I understand 'emergence' it arises because things have properties and emergence arises in very special particular cases of combinations of these properties.

[Genady]

9 minutes ago, studiot said:

emergent from what ?

As I understand 'emergence' it arises because things have properties and emergence arises in very special particular cases of combinations of these properties.

I understand 'emergence' in the same way. I think of this emergence as a result of averaging uncountable local changes in curvature, expansions and contractions caused by dynamics of local mass-energy inhomogeneities. On the large scales where these inhomogeneities become 'invisible', the net effect of all these local effects becomes uniform expansion of space.

Edited September 2, 2023 by Genady

[studiot]

37 minutes ago, Genady said:

I understand 'emergence' in the same way. I think of this emergence as a result of averaging uncountable local changes in curvature, expansions and contractions caused by dynamics of local mass-energy inhomogeneities. On the large scales where these inhomogeneities become 'invisible', the net effect of all these local effects becomes uniform expansion of space.

Wouldn't this contradict Gauss' shell-flux  theorem ?

[Genady]

4 minutes ago, studiot said:

Wouldn't this contradict Gauss' shell-flux  theorem ?

I don't see why it would.

(We cannot use Newton's law of gravity here. Expansion appears only in the Einstein field equation.)

[grayson]

If we take into account of the cosmological constant, it is an emerging theory. We have no true idea of why dark matter exists. In my mind I think it is quantum fluctuations but I could be wrong.

emerging property, not theory 

sorry

[studiot]

1 hour ago, Genady said:

I don't see why it would.

(We cannot use Newton's law of gravity here. Expansion appears only in the Einstein field equation.)

One of the consequences of Gauss is that the view from outside a shell is independent of the disposition of the flux charge within the shell.

Newton is not required for this and it also applies to fluxes other than gravity.

In turn this leads to multiple, if not infinite, solutions.

I see it as bit like the different between the moment of inertia and product of inertia.

There are an infinity of solutions to question what disposition of mass yields a given moment of inertia, but only one to the question what yield a given product of inertia.

Somewhere there is more to the story.

[Genady]

5 minutes ago, studiot said:

One of the consequences of Gauss is that the view from outside a shell is independent of the disposition of the flux charge within the shell.

Newton is not required for this and it also applies to fluxes other than gravity.

In turn this leads to multiple, if not infinite, solutions.

I see it as bit like the different between the moment of inertia and product of inertia.

There are an infinity of solutions to question what disposition of mass yields a given moment of inertia, but only one to the question what yield a given product of inertia.

Somewhere there is more to the story.

I don't know how to apply Gauss in this case. When gravity is described by a non-flat spacetime metric, what is the 'flux'?

[studiot]

35 minutes ago, Genady said:

I don't know how to apply Gauss in this case. When gravity is described by a non-flat spacetime metric, what is the 'flux'?

Difficult because Gauss require a closed n dimension 'shell'.

You can describe a closed sperical shell, which is 2D and curves in 3D.

But it is not possible to surround an object in 4d with a corresponding 3D 'shell'.

There is always a route through the 4th D from inside to outside.

 

Flux refers to contours or imaginary equipollent lines in the normal way connecting points in the manifold.

[TheVat]

2 hours ago, Genady said:

These need to be considered separately. Vacuum energy is not a model of expansion. GR alone models expansion. Vacuum energy models acceleration of expansion.

And in fact, effect of vacuum energy on the cosmological scales would be acceleration of expansion while on lab scales it is something else, an extra repulsion unrelated to any expansion.

I was thinking of dark energy theory, where expansion and vacuum energy are related.  As in this...

https://www.cambridge.org/core/journals/journal-of-plasma-physics/article/abs/dark-energy-and-dark-matter-as-due-to-zero-point-energy/DFAC4A98338A39DDFD72DA85A6B09F06

An attempt is made to explain dark energy and dark matter of the expanding universe in terms of the zero point vacuum energy. This analysis is mainly limited to later stages of an observable nearly flat universe. It is based on a revised formulation of the spectral distribution of the zero point energy, for an ensemble in a defined statistical equilibrium having finite total energy density. The steady and dynamic states are studied for a spherical cloud of zero point energy photons. The ‘antigravitational’ force due to its pressure gradient then represents dark energy, and its gravitational force due to the energy density represents dark matter. Four fundamental results come out of the theory. First, the lack of emitted radiation becomes reconcilable with the concepts of dark energy and dark matter. Second, the crucial coincidence problem of equal orders of magnitude of mass density and vacuum energy density cannot be explained by the cosmological constant, but is resolved by the present variable concepts, which originate from the same photon gas balance. Third, the present approach becomes reconcilable with cosmical dimensions and with the radius of the observable universe. Fourth, the deduced acceleration of the expansion agrees with the observed one. In addition, mass polarity of a generalized gravitation law for matter and antimatter is proposed as a source of dark flow.

 

[Genady]

3 minutes ago, TheVat said:

I was thinking of dark energy theory, where expansion and vacuum energy are related.  As in this...

https://www.cambridge.org/core/journals/journal-of-plasma-physics/article/abs/dark-energy-and-dark-matter-as-due-to-zero-point-energy/DFAC4A98338A39DDFD72DA85A6B09F06

An attempt is made to explain dark energy and dark matter of the expanding universe in terms of the zero point vacuum energy. This analysis is mainly limited to later stages of an observable nearly flat universe. It is based on a revised formulation of the spectral distribution of the zero point energy, for an ensemble in a defined statistical equilibrium having finite total energy density. The steady and dynamic states are studied for a spherical cloud of zero point energy photons. The ‘antigravitational’ force due to its pressure gradient then represents dark energy, and its gravitational force due to the energy density represents dark matter. Four fundamental results come out of the theory. First, the lack of emitted radiation becomes reconcilable with the concepts of dark energy and dark matter. Second, the crucial coincidence problem of equal orders of magnitude of mass density and vacuum energy density cannot be explained by the cosmological constant, but is resolved by the present variable concepts, which originate from the same photon gas balance. Third, the present approach becomes reconcilable with cosmical dimensions and with the radius of the observable universe. Fourth, the deduced acceleration of the expansion agrees with the observed one. In addition, mass polarity of a generalized gravitation law for matter and antimatter is proposed as a source of dark flow.

 

I don't understand why it is needed. In GR, a universe homogeneously filled with inert dust, expands.

My question is, in a universe with local inhomogeneities of such dust and thus not necessarily expanding locally, is the large scales expansion an emergent property?

[TheVat]

I guess I was trying to ask, with my limited physics, if it could be an intrinsic property at the smaller scale which is just masked by the gravitational attraction of local inhomogeneous clumps so it's only noticeable at the largest scale.  Maybe it doesn't make sense outside of QFT?  Can zero point energy be the cosmological constant?

Edited September 2, 2023 by TheVat

[Genady]

3 minutes ago, TheVat said:

I guess I was trying to ask, with my limited physics, if it could be an intrinsic property at the smaller scale which is just masked by the gravitational attraction of local inhomogeneous clumps so it's only noticeable at the largest scale.  Maybe it doesn't make sense outside of QFT?  Can zero point energy be the cosmological constant?

It could be an intrinsic property which adds to the expansion, but the large-scale expansion would happen without it anyway. The large-scale expansion happens without cosmological constant either.

[TheVat]

4 minutes ago, Genady said:

It could be an intrinsic property which adds to the expansion, but the large-scale expansion would happen without it anyway. The large-scale expansion happens without cosmological constant either.

Is that certain?  Why is it not just as valid to have a model in which space does not expand and objects simply move apart under the influence of their mutual gravity and dark energy repulsion?  In that case, nothing emerges at large scale, it just becomes easier to observe because you don't have local energy densities obscuring it.

 

[Genady]

6 minutes ago, TheVat said:

Is that certain?  Why is it not just as valid to have a model in which space does not expand and objects simply move apart under the influence of their mutual gravity and dark energy repulsion?  In that case, nothing emerges at large scale, it just becomes easier to observe because you don't have local energy densities obscuring it.

 

It is not valid because GR does not allow it. Homogeneous isotropic space cannot be stable. That's why the prediction of expanding space happened a few years before Hubble's observations.

[TheVat]

Ok, that helps.  Thanks.